Nisin, an antimicrobial substance produced by certain strains of Streptococcus lactis, has known food preservative use. It has the property of inhibiting the growth of certain Gram-positive bacteria, but not Gram-negative ones or yeasts or molds. A further, and most important practical property, lies in the ability of Nisin to prevent the outgrowth of germinated bacterial spores. The commercial use of Nisin to date very much relates to the property of preventing the outgrowth of germinated bacterial spores, in some applications that involves the prevention of spoilage in foods which have received at least sufficient heat treatment to destroy vegetative bacterial cells. Another classic use has been for the prevention of clostridial spoilage in process cheese products, which receive a less severe heat treatment.
The use of Nisin in process cheese products has been the subject of earlier patents; see British Pat. No. 713,251 and U.S. Pat. No. 2,744,827 dated 1954 and 1956, respectively. These patents relate to the manufacture or preservation of cheese of kinds liable to become blown or distended due to the growth of spoilage organisms which are anaerobic sporeformers, typically bacteria known as Clostridia. They also relate to process cheese which is liable to be spoiled as above. The levels of Nisin referred to in these patents are in the range of 50 to 375 units per gram. One "unit" is equivalent to 1 International Unit (I.U.) of Nisin activity and is equivalent to 0.025 microgram of Nisin.
Until recent times it was not considered that process cheese products would be liable to spoilage by Clostridium botulinum organisms or that the production of the associated toxin would present a potential health risk. Recent findings however are that such risks can exist, particularly in process cheese formulations where the water content is in excess of 54 percent. See Kautter et al "Toxin Production by Clostridium botulinum in Shelf-Stable Pasteurized Process Cheese Spreads", Journal of Food Protection, 42 pp. 784-786 (1979); Scott and Taylor, "Effect of Nisin on the Outgrowth of Clostridium botulinum Spores", J. Food Sci., 46(1) pp. 117-120 (1981); Scott and Taylor, "Temperature, pH, and Spore Load Effects on the Ability of Nisin to Prevent on the Outgrowth of Clostridium botulinum Spores", J. Food Sci. 46(1): 121-126 (1981); and Somers and Taylor, Research Note--"Further Studies on the Antibotulinal Effectiveness of Nisin in Acidic Media", J. Food Sci., 46(6): p. 1972-3.
Our studies at the fundamental level have shown that Nisin does have the ability to inhibit growth of botulinum spores from different culture and type sources. The significant finding, however, is that the levels of Nisin required to effect complete inhibition are substantially higher for Clostridium botulinum than for other non-pathogenic clostridia or aerobic sporeformers commonly encountered as food spoilage organisms. We have further demonstrated in a protracted shelf-life study with high-moisture, reduced sodium process cheese spread that complete inhibition of Clostridium botulinum spore outgrowth is achieved at a Nisin addition level of 250 ppm (10,000 IU nisin/gram). This Nisin level is 20 times the maximum addition rate in current commercial practice.